The draft beer industry employs re-usable aluminum kegs to distribute draft beer. The kegs are simple aluminum vessels that can be filled with beer, pressurized, and then the beer flows out of a top-mounted valve/spout. The keg typically embodies no technology beyond a simple pressure valve/spout on the top.
Kegs are often designed with a spherical round bottom that is then surrounded or collared with a round aluminum sheath that allows the keg to sit upright and level. This collar on the bottom of the keg, combined with the spherical round bottom, creates a constructed void or space under the keg. All kegs have this empty space under the main container portion.